Method of using a xeroradiographic plate which is insensitive to visible light

ABSTRACT

A method of imaging a xeroradiographic plate is disclosed. The method comprises rendering said plate insensitive to visible light by applying an electrostatic charge and exposing the plate to X-ray radiation to form an electrostatic charge pattern.

United States Patent [7 2 Inventor Paul J. Regensburger West Webster,N.Y.

[21 Appl. No. 870,975

[22] Filed June 25, 1969 [23] Division of Ser. No. 527,904, Feb. 116,

1966, Patent No. 3,501,343

[45] Patented Nov. 16, 1971 [73] Assignee Xerox Corporation Rochester,N.Y.

[54] METHOD OF USING A XERORADIOGRAPHIC PLATE WHICH IS INSENSITIVE T0VISIBLE LIGHT 4 Claims, 1 Drawing Fig.

521 user 250/65 ZE, 250/495 zc Primary Examiner-James W. LawrenceAssistant Examiner-C. E. Church Attorneys-Stanley 2. Cole and James J.Ralabate ABSTRACT: A method ofimaging a xeroradiographic plate isdisclosed. The method comprises rendering said plate insensitive tovisible light by applying an electrostatic charge and exposing the plateto X-ray radiation to form an electrostatic charge pattern.

PATENTEDNUV 16 I971 3,621 p248 INVENTOR. PAUL J. REGENSBURGER BY 0M0ATTORNEY METHOD OF USING A XERORADIOGRAPHIC PLATE WHICH IS INSENSITIVE TVISIBLE LIGHT This application is a division of application, Ser. No.527,904, filed Feb. 16, 1966.

This invention relates in general to xerography and, more specifically,to a system utilizing an improved xeroradiographic plate.

In the art of xerographic copying, as originally disclosed by Carlson inU.S. Pat. No. 2,297,691, and as further described by many relatespatents in the field, a xerographic plate containing a photoconductiveinsulating layer is first given a uniform electrostatic charge in orderto sensitize its entire surface. The plate is then exposed t6 an imageof activating elec tromagnetic radiation such as light, X-rny, or thelike which selectively dissipates the charge in the illuminated areas ofthe photoconductive insulator while leaving behind a latente|ectrostatic image in the nonilluminated areas. This latentelectrostatic image is then developed and made visible by depositingfinely divided, electroscopic marking material on the surface of hephotoconductive insulating layer. Where reusable photoconductiveinsulating material is used, the visible image formed by theelectroscopic marking material is transferred to a second surface, suchas a sheet of paper, and fixed in lace thereon to form a permanentvisible reproduction of the original image. Where a nonreusablephotoconductive insulating material is used, the marking material ortoner particles are directly fixed in place onthe surface of thenonreusable insulating material thereby eliminating the transfer stepfrom the previous process.

Presently, high quality xerographic plates employing a photoconductivelayer of amorphous or vitreous selenium are being used with greatsuccess. These plates are illustrated in US. Pat. No. 2,970,906 toBixby. The amorphous selenium plate is responsive to both visible lightand the X-ray portion of the electromagnetic spectrum. As a result ofthis wide spectral range of light sensitivity, the plates must becarefully han dlcd and protected from visible light when they are usedfor X- ray imaging. It can therefore be seen that selenium plates whichare to be used in X-ray xerography, hereinafter referred to asxeroradiography, must be stored and used under dark room conditions.There is, therefore, a need for xeroradiographic plates which areinsensitive to visible light while still being responsive to x rayradiation. Such a plate having the properties of light insensitivitywould permit continuous observation of the X'ray process under normalillumination and thus eliminate the need for the additional apparatusrequired to maintain dark room conditions.

It is, therefore, an object of this invention to provide an improvedxeroradiographic plate and an improved method for preparing said platewhich overcomes the above noted disadvantages.

It is another object of this invention to provide a xeroradio graphicplate which is substantially insensitive to visible light.

It is a further object of this invention to provide a method forpreparing a xeroradiographic plate which is substantially insensitive tovisible light.

The foregoing objects and others are accomplished in accordance withthis invention by preparing a xeroradiographic plate which is madesubstantially insensitive to light by depositing a thin coating of about1 to microns oflow carrier range selenium or a thin coating of seleniumdoped with a material having a low carrier range onto normal selenium orsome other photoconductor such as those disclosed in US. Pats. Nos.2,803,542, 3,121,006, 3,121,007, and 3,151,982 to Ullrich, Middleton etal., Middleton et al., and Corrsin, respectively. The term normalselenium means that most of the carriers are able to move across acharged selenium layer when exposed to activating electromagneticradiation. Normal selenium will hereinafter be referred to asconventional selenium or simply selenium.

By way of illustration, with no intent to limit the method of preparinga conventional selenium plate, a selenium plate may be prepared in thefollowing manner: A metalsupport backing plate such as brass or aluminumis cleaned and then positioned in a vacuum chamber with a temperaturecontrol means for the backing plate, such as for example, a contactingtemperature control platen provided with water cool means. The vacuumchamber is then evacuated to a low pressure, such as about 1 micron ofmercury, or less. A layer of high-purity (99.99%+) selenium is thenevaporated at a temperature of about 280 to 300 C. onto the plate. Theselenium may be placed in any inert refractory container such asmolybdenum, pyrex, or the like. Common thicknesses for selenium platesrange from 20 to microns. When used for X-ray imaging or otherhigh-energy electromagnetic radiation, much thicker layers on the orderof to 500 microns may be employed to take advantage of the increasedstopping power of the thicker layer when exposed to the high-energypenetrating radiation.

The advantages of the improved xeroradiographic plate and the method forproducing said plate will become more apparent upon the followingdisclosure of the invention and when taken in conjunction with theaccompanying drawing wherein:

The drawing is a schematic sectional view of one form ofxeroradiographic plate according to the invention.

In the drawing reference character 10 designates an embodiment of onexeroradiographic plate according to the invention. This plate has aconventional electrically conductive support member 11 such as brass,aluminum, steel or the like. The support member may be of any convenientthickness, rigid or flexible, and may be in the form of a sheet, web,cylinder or the like. it may also comprise other materials such asmetallized paper, plastic sheets covered with a thin coating of aluminumor copper iodide or glass coated with a thin layer of chromium or tinoxide. Reference character 12 designates a conventional photoconductiveinsulating layer such as amorphous selenium, selenium and arsenic,selenium with photoconductive particles dispersed therein, or anonphotoconductive binder saturated with photoconductive particles. Thislayer commonly ranges from about 20 to 100 microns for normalxerographic applications and may be as high as 500 microns for X-rayapplication. Reference character 13 represents a thin outer layerofselenium having a low carrier range or selenium doped with an additivethus having a low carrier range. The thickness of this layer may rangefrom about 1 to 20 microns, and usually ranges up to about 10 percent ofthe thickness of the selenium substrate 12.

This invention contemplates the use of a thin coating or layer of lowcarrier range selenium or low carrier range selenium formed by dopingwith a halogen or thallium on a conventional selenium plate such as thatdescribed above.

When the plate is formed by the application of a thin layer of low rangeselenium, said layer may be formed by evaporating the thin layer ofconventional selenium onto positive relatively cold substrate of aconventional selenium plate. Temperatures in the range of 15 to 35 C.have been found to provide a satisfactory substrate for the evaporationof the thin outer layer which exhibits a low carrier range when chargedto a positive voltage. if the temperature exceeds about 35 C., the thinselenium layer looses its low carrier range properties and becomessensitive to light similar to the normal conventional selenium substrateupon which it is coated. A suitable thickness for the outer layer of low:range selenium is in the range of l to 20 microns. The formation of thethin layer of selenium on the conventional selenium plate under theseconditions results in a thin layer of low range selenium on a thickerbase layer of conventional selenium. When charged positive, the surfaceof this plate is insensitive to light and when exposed to X-rayradiation, it creates carries throughout the bulk of the conventionalselenium layer, with the carriers so created, moving easily to thesurface and thus discharging the plate. The areas unexposed to X-rayradiation remain charged, and the latent electrostatic image can bedeveloped in the normal xerographic manner.

When the light-insensitive plate is formed by doping with a thin layerof thallium, a prealloy mixture of thallium and highpurity selenium isplaced in a molybdenum boat in a vacuum chamber and vacuum evaporatedonto a normal selenium plate such as has already been described. Thethickness of the thallium mixture may be of any convenient thicknesssuch as from 1 to 20 microns. The concentration of the thallium rangesanywhere from a few parts per million to 10,000 parts per million.Although the concentration of thallium may range widely, it is usuallyconvenient to maintain the concentration from to 1,000 parts per millionwith the amount of thallium used being only that necessary to render theplate insensitive to light. When using the thallium doped layer, it isnecessary to charge the plate positive in order to render it insensitiveto light.

When halogen is used as the dopant to form the low carrier range layer,the concentration of the halogen may be controlled by mixing the desiredconcentration from master alloys which may be made in the laboratory bystandard techniques. Concentrations from the few parts per million up to10,000 parts per million are effective with the amount of dopant usedbeing only that necessary to render the plate insensitive to light.Ranges of 10 to 1,000 parts per million are normally suitable to renderthe plate insensitive to light. The total thickness of the outer coatingis conveniently from 1 to 20 microns. To render the halogen coatedplates insensitive to light, it is necessary that they be negativelycharged at their surface.

' The thickness of the low carrier range selenium layer formed either byevaporating on a cold substrate or by doping with a suitable materialneed only be as thick as that necessary to render the plate insensitiveto light. Thickness of from 1 to 20 microns have proven satisfactory,but this range is not intended to be limiting as layers outside thisrange are also effective. in the doped low carrier range seleniumlayers, the concentration of the thallium and the halogen need not onlybe present in amounts necessary to render the layer insensitive tovisible light while still being sensitive to high-energy penetratingradiation, outside the visible spectral range. If, for example, undergiven conditions, 1,000 parts per million are effective to render aplate insensitive to light then it would be necessary and uneconomicalto use 10,000 parts per million. When the low range layer is formed on acold substrate, the layer thus formed exhibits low range carrierproperties at forming temperatures below about 35 C. Any convenienttemperature below 35 C. appears to be satisfactory with 10 to 35 C.being preferred in that temperature above about 35 C. result in theevaporated layer being light sensitive,'while temperatures below about10 C. require special apparatus to insure that the cold substrate isaccurately maintained at temperature.

The following examples further specifically define the present inventionwith respect to the method of making double layered xeroradiographicplates which are insensitive to visible light. The parts and percentagesin the disclosure are by weight unless otherwise indicated. The examplesbelow are intended to illustrate various preferred embodiments of thisinvention.

EXAMPLE I A conventional selenium plate is made by placing a brassbacking plate on a water-cooled support member within a vacuum chamber Asource of high-purity selenium is placed in a Pyrex boat below the brassplate. The chamber is then evacuated to a pressure of about 0.5 micronof mercury. The Pyrex boat is heated to a temperature of about 280 C. toevaporate a uniform amorphous coating of selenium to a thickness ofabout 100 microns onto the brass plate. During the evaporation step thebrass plate is maintained at a temperature of about 60 C. Aftercompletion of the evaporation step, the thus-formed plate is chargedpositively with a corona discharge electrode to a voltage of about 600volts. The entire plate is then exposed to light. The previously chargedsurface is substantially discharged by the light exposure, rendering theplate substantially insensitive to X-ray radiation. The plate is againcharged to a voltage of 600 volts and kept under dark room conditions.While unexposed to light the plate is subjected to a pattern ofactivating X-ray radiation whereby the electrostatic surface charge isdissipated in the areas energized by the X-ray radiation, while theareas unexposed to the radiation remain unefi'ected, forming a latentelectrostatic image which is developed in any conventional xerographicmanner.

EXAMPLE ll A l00-micron plate of selenium on brass is made in theconventional manner already described above in Example I. This plate isthen placed in a vacuum chamber on a water cooled support platen. Asource of high-purity selenium is then placed in a molybdenum boat inthe vacuum chamber below the selenium plate. The chamber is thenevacuated to a pressure of about 0.5 micron of mercury and themolybdenym boat maintained at a temperature of about 280 C. to evaporatea S-micron coating of selenium onto the selenium plate which ismaintained at a temperature of about 15 C. After the completion of theevaporation step and cooling, the vacuum is broken and the plate isremoved from the vacuum chamber. The double-layered selenium plate isthen charged to a positive polarity with a corona discharge electrode,as described in U.S. Pat. No. 2,777,957 to Walkup, to a voltage of about600 volts. The resulting plate is substantially insensitive to visiblelight. This plate is then exposed to X-ray activating radiation wherebythe electrostatic charge on the surface is dissipated in the areasenergized by the X-ray radiation while the areas unexposed to the X-raysremain unetfected, forming a latent electrostatic image which can bedeveloped in the normal xerographic manner.

EXAMPLE ll! A l60-micron plate of selenium on aluminum is made in theconventional manner described in example 1. A master alloy ofhigh-purity selenium containing 1,000 parts per million of thallium isplaced in an inert Pyrex evaporation boat in the vacuum chamber. Thevacuum chamber is then sealed and evacuated to a pressure ofapproximately 0.5 micron of mercury while the aluminum plate is held ata temperature of about 70 C. The Pyrex boat is heated to a temperatureof about 280 C. to evaporate a 4-micron coating of the selenium-thalliummixture onto the selenium plate. After completion of the evaporationstep the vacuum is broken and the chamber cooled to room temperature.The double-layer plate is moved from the evacuation chamber and chargedpositively to about 700 volts with a corona discharge electrode asdescribed in example I]. This plate is insensitive to visible light andreadily responds to penetrating radiation of the X-ray type as the platedescribed in example ll.

EXAMPLE IV A l02-micron plate of conventional selenium or brass is madein the manner described in example I. A master alloy of 500 parts permillion chlorine in high-purity selenium is placed in an inert Pyrexevaporation boat in a evacuation chamber as described in example 11. Theselenium chlorine mixture is evaporated to a thickness of 3 microns ontothe selenium plate under conditions similar to that set forth in examplelll. The resulting double-layer plate is then charged negative to avoltage of 600 volts as described in example ll above. This plate isinsensitive to visible light, but responds well when exposed to X-rayradiation.

EXAMPLE V A ZOO-micron plate of conventional selenium on brass is madein the manner described in example 1. A master alloy of 500 parts permillion of bromine in high-purity selenium is placed in an inert Pyrexevaporation boat in an evacuation chamber and evaporated to a thicknessof 4 microns onto the conventional selenium plate under the conditionsdescribed in example Ill. The resulting plate is insensitive to visiblelight and responds well when exposed to X-ray radiation.

EXAMPLE VI A ISO-micron plate of conventional selenium on aluminum ismade in the manner described in example i. A master alloy of 1,000 partsper million of iodine and high-purity selenium is placed in an inertPyrex evaporation boat in an evacuation chamber as described in exampleIll. The selenium-iodine mixture is evaporated to a thickness of micronsonto the conventional selenium plate under the conditions set forth inexample Hi. This plate is insensitive to visible light and readilyresponds to penetrating radiation of the X-ray type as the platesdescribed in examples lI-V.

The plates made in accordance with the present invention are normallyused in a xeroradiographic process which includes at least the threebasic steps of charging, exposing, and developing. if the plate iscoated with low range selenium evaporated at less than about 35 C. or isdoped with thallium, the plate must be charged to a positive polarity inorder to render it insensitive to light. On the other hand, if the lowcarrier range outer coating is doped with halogen, the plate must bechanged to a negative polarity in order to render it light insensitive.The plates then exposed to X-ray radiation which selectively dissipatesthe previously applied charge. Through the use of these plates as setforth in the present invention the need for dark room conditions isentirely eliminated.

Although specific components, proportions, and procedures have beenstated in the above description of the preferred embodiments of theinvention other suitable materi als, as listed above, may be used withsimilar results. in addition, other materials and procedures may bevaried to synergize, enhance, or otherwise modify the end result. Forexample, during the evaporation step in the manufacturing of theconventional selenium plate, the low carrier range outer coating may beevaporated onto the freshly made conventional selenium plate immediatelyupon the termination of the evaporation of the conventional seleniumlayer. Also, the alloy mixtures for the low carrier range coatings, maybe made from commercially available master alloys or prepared in thelaboratory by conventional means such as simply mixing the appropriateamount of additive element with the high-purity selenium.

Other modifications and ramifications of the present invention willoccur to those skilled in the art upon a reading of the disclosure.These are intended to be included within the scope of this invention.

What is claimed is:

l. A method of forming a latent image on a light-insensitivexeroradiographic plate which comprises rendering said plate insensitiveto visible light by applying an electrostatic charge to an outer layerof low carrier range photoconductive material selected from the groupconsisting essentially of thalliumdoped selenium and halogen-dopedselenium which overlays a layer of photoconductive material having ahigh carrier range, and exposing said charged layer to a pattern of)t-ray radiation, thereby forming a latent electrostatic image on saidouter layer.

2. The method of claim it wherein the latent electrostatic image isdeveloped to make the pattern visible.

3. The method of claim 2 wherein a positive electrostatic charge isapplied to a low carrier range outer layer comprising thallium-dopedselenium.

4 A method of claim 2 wherein a negative electrostatic charge is appliedto a low carrier range outer layer comprising halogen-doped selenium.

1. A method of forming a latent image on a light-insensitivexeroradiographic plate which comprises rendering said plate insensitiveto visible light by applying an electrostatic charge to an outer layerof low carrier range photoconductive material selected from the groupconsisting essentially of thallium-doped selenium and halogen-dopedselenium which overlays a layer of photoconductive material having ahigh carrier range, and exposing said charged layer to a pattern ofX-ray radiation, thereby forming a latent electrostatic image on saidouter layer.
 2. The method of claim 1 wherein the latent electrostaticimage is developed to make the pattern visible.
 3. The method of claim 2wherein a positive electrostatic charge is applied to a low carrierrange outer layer comprising thallium-doped selenium.
 4. A method ofclaim 2 wherein a negative electrostatic charge is applied to a lowcarrier range outer layer comprising halogen-doped selenium.